Transparent resin-coated stainless steel article

ABSTRACT

A stainless steel article which has a transparent and uniform resin coating thereon, which can utilize the surface appearance of the stainless steel used as a substrate, which shows a high adhesive strength between the resin coating and stainless steel as a ground, which can be easily produced with a small number of steps, and which is particularly useful as an automotive exterior article. The stainless steel article comprises a stainless steel substrate and a transparent resin film with which at least one surface of the substrate is coated, the resin film comprising a transparent synthetic resin base and a transparent heat-sensitive adhesive layer formed on the synthetic resin base on the side of the stainless steel substrate, and the resin film being bonded to the stainless steel substrate through a silane coupling agent.

TECHNICAL FIELD

The present invention relates to a stainless steel article, inparticular to a transparent resin-coated stainless steel articleexcellent in water resistance, light resistance and the like, andcapable of manifesting the features of the ground of stainless steelwithout modification. The transparent resin-coated stainless steelarticle of the present invention can be advantageously used as, forexample, an automotive exterior part etc.

BACKGROUND OF THE INVENTION

Stainless steel sheets have heretofore been excellent in durability andformability, and have a metallic appearance. Accordingly, they have beenwidely used as exterior materials in various technological fields.

For example, in the field of the automobile industry, a stainless steelsheet is used as an exterior part in a molding subsequent to rollforming and in a pillar subsequent to press forming, because theexterior part made of a stainless steel sheet can give an artisticappearance in which the unique brightness and feeling of metallic massthat the stainless steel sheet has are utilized by coating the steelsheet with a transparent resin.

However, it is difficult to readily form a transparent resin coating ina satisfactory form on the surface of such a stainless steel sheetexterior part. For example, in the method of coating a stainless steelsheet with a molten resin by extrusion, a resin having a specificcomposition suited to the extrusion conditions must be used. Moreover,the steel sheet surface must be thickly coated with the resin. It istherefore impossible to thinly coat an exterior part having acomplicated shape with the resin.

It has also been proposed to form a resin coating by painting in amolten resin. For example, Japanese Unexamined Patent Publication(Kokai) No. 62-174387 discloses a method of forming a fluoro resincoating excellent in weathering resistance, resistance to dirt andslipperiness on the surface of a stainless steel. In order to improvethe adhesive strength of the fluoro resin with the stainless steel, themethod of forming a resin coating comprises coating the surface of thestainless steel with a silane coupling agent having an amino group, in acoating amount of 1 to 500 g/m². Moreover, Japanese Unexamined PatentPublication (Kokai) No. 63-260965 discloses a transparent resincomposition for stainless steel in which 0.1 to 5% by weight of aterminal epoxy group-containing silane coupling agent and 0.5 to 10parts by weight of an oxalanilide ultraviolet absorber are added to 100parts by weight of a resin containing 60 to 99% by weight of a solidcomponent of a solvent-soluble fluoro resin having a hydroxyl value of20 to 250 and 1 to 40% by weight of a solid component of a curing agent.

However, in the resin coating method in which a silane coupling agent isused, as described above, expected action and effect cannot be obtainedso long as a silane coupling agent having a special composition is notemployed. Moreover, in these methods, forming a resin coating having auniform thickness is difficult, and articles having the shape of finalproducts must be coated with a resin and baked individually. As aresult, productivity and efficiency are reduced.

Furthermore, coating the surface of a stainless steel sheet with a resinfilm in place of extruding a molten resin or forming a resin coating hasalso been proposed (for example, refer to Japanese Unexamined PatentPublication (Kokai) No. 2-234859). The method of using a resin film canrealize the following: easy formation of a coating having a uniformthickness; provision of a product having an appearance of high quality;and continuous lamination of a resin film to a flat sheet of a stainlesssteel and subsequent forming such as roll forming and press forming.Accordingly, the method also has the advantage of increasedproductivity. However, the method of laminating a resin film to astainless steel sheet using a pressure-sensitive adhesive and aheat-sensitive adhesive restricts the properties (such as adhesivestrength) of the adhesive to be used. The method therefore cannot beadvantageously used for specific applications. For example, a stainlesssteel article used as an automotive exterior part must pass variousendurance tests. However, the adhesive subsequent to the test can nolonger show an adhesive strength comparable to that prior to the test.Accordingly, the stainless steel article with a resin film cannot beused as an automotive exterior part without further processing.

In order to supplement such a low adhesive strength between thestainless steel article and the resin film, there is a common methodcomprising continuously coating the surface of the stainless steelarticle with an acrylic primer and baking the primer in the processingsteps (of Japanese Unexamined Patent Publication (Kokai) No. 2-234859,if. necessary). However, since the acrylic primer has a poor lightresistance, the primer tends to peel off at the interface between thestainless steel and the primer when the primer is exposed to light overa long period of time. In order to avoid such problems produced by theacrylic primer, a transparent resin film excellent in light transmissioncannot be used on a stainless steel article. Presently, stainless steelarticles are coated by extrusion of polyvinyl chloride (PVC) or acolored resin film having a toning level sufficient to substantiallyprevent transmission of light. Thus it would be an advantage to providea stainless steel as an automotive exterior article on which atransparent decorative film is formed.

SUMMARY OF THE INVENTION

The present invention is a stainless steel article which has atransparent and uniform resin coating formed thereon. The articleutilizes the surface appearance of the stainless steel substrate, andpossesses a high adhesive strength between the resin coating andstainless steel. This article can be produced with a limited number ofsteps, and is particularly useful as an automotive exterior article.

According to the present invention, a stainless steel substrate iscoated with a resin film on at least one surface of the substrate. Theresin film includes a synthetic resin base and a heat-sensitive adhesivelayer formed on the synthetic resin base on the side of the stainlesssteel substrate. The resin film is bonded to the stainless steelsubstrate through a silane coupling agent.

DETAILED DESCRIPTION

The transparent resin-coated stainless steel article according to thepresent invention comprises a stainless steel substrate and atransparent resin film with which at least one surface of the substrateis coated. The stainless steel article may optionally have additionallayers or films for improving the properties and appearance.

There is no specific limitation on the stainless steel substrate in theresin-coated stainless steel article of the present invention. Thesubstrate can be formed to have various shapes from an optionallyselected and commercially available stainless steel. In general, thesubstrate can be formed from an austenitic, ferritic or martensiticstainless steel containing at least about 10% by weight of chromium forthe following reasons. When the chromium content in the stainless steelis large, the steel can be readily formed to have a predetermined shape.Specific types of suitable stainless steels include SUS 430 and SUS 304.Foreign materials such as dust are usually removed from the surface ofsuch a stainless steel. The stainless steel may then be subjected toconventional treatment such as surface treatment. Examples of thesurface treatment include cleaning the stainless steel surface withalcohol for removing fats and oils and passivation thereof forpreventing rusting. The stainless steel may optionally be subjected tochemical conversion treatment. Moreover, the stainless steel can usuallybe obtained as a sheet having a thickness selected from a wide thicknessrange.

A transparent resin film is then continuously laminated to the steelsheet according to the present invention, and the steel sheet issubjected to an various forming procedures such as roll forming or pressforming to give an article having a desired shape. As explained below,the resin-coated stainless steel article of the present invention isuseful for an automotive exterior part, such as a molding or a pillar.

The transparent resin film with which the stainless steel substrate asexplained above is generally coated with a transparent synthetic resinbase and a transparent heat-sensitive adhesive layer formed on thestainless steel substrate side of the base.

The transparent resin film can be formed from any of the varioustransparent synthetic resin materials to have a freely selectedthickness so long as the film does not exert adverse effects on theappearance etc. of the stainless steel substrate used as an underlayer.In addition, the term “transparent” herein naturally includes acompletely transparent state with a transmittance of visible light of100%, but also includes a state showing a lower transmittance of visiblelight in accordance with desired action and effect. That is, the resinfilm used in the present invention is satisfactory when it has atransparency to such a degree that it improves the appearance of theground stainless steel. The resin film therefore preferably has atransmittance of visible light of 50% or more. Moreover, the resin filmmay optionally be transparent and colored.

Examples of the synthetic resin material suitable for the resin filminclude polyurethane resin, acrylic resin, polyester resin, fluoroacrylic resin and polyvinyl chloride resin, although the suitablesynthetic resin material is not restricted to those mentioned above.Moreover, in order to improve the properties of the resin film, thoseresins may contain additives such as an ultraviolet absorber and acoloring agent.

Although the transparent resin film is usually used in a single layer,it may optionally be used in a multi-layer structure of two or morelayers. Moreover, an overcoat of polyurethane clear etc. may be formedon the resin film. Although the film thickness of the transparent resinfilm can be altered in a wide range in accordance with a desired effector the like, the thickness is usually in a range of about 30 to 1,000μm, preferably about 50 to 300 μm.

Since the transparent resin film is laminated to the stainless steelsubstrate, a transparent heat-sensitive adhesive layer is necessary, andthe layer is formed on the stainless steel substrate side of thesynthetic resin base. Conventionally available transparentheat-sensitive adhesives are suitable for use with the presentinvention. Examples of the adhesive include polyurethane adhesives,polyester adhesives, olefinic adhesives, acrylic adhesives and vinylchloride adhesives. The definition of “transparent” explained above isalso applied to the transparency of such heat-sensitive adhesives.Although the thickness of the heat-sensitive adhesive layer may bevaried in accordance with the adhesive strength required between thestainless steel substrate and the transparent resin film, the thicknessis usually about 5 to 500 μm, preferably about 30 to 200 μm.

The heat-sensitive adhesive layer as described above is preferablyformed by adjusting the type and film thickness of the adhesive so thatthe adhesive strength of the transparent resin film acting on thestainless steel substrate becomes 14 N/25 mm or more in terms of peelingstrength. The adhesive strength thereof acting on the stainless steelsubstrate is more preferably 20 N/25 mm or more in terms of peelingstrength. When the peeling strength of the resin film from the stainlesssteel substrate is 14 N/25 mm or more, the article used as an automotiveexterior part can be stably used over a long period of time withoutfailure of the adhesive. When the peeling strength lowers 14 N/25 mm,there is a possibility that the resin film peels off due to aging.

For the stainless steel article of the present invention, thetransparent resin film is not directly laminated to the stainless steelsubstrate, but laminated thereto through a silane coupling agent. Thatis, the use of a silane coupling agent as a primer achieves an excellentbonding condition between the resin film and the stainless steelsubstrate.

The silane coupling agent used herein as a primer may be basically anytype known in silicone chemistry. Accordingly, the silane coupling agentcan be represented by the general formula:Y˜SiX₃wherein Y is a reactive organic functional group such as an amino group,an epoxy group, a vinyl group, an (meth)acrylic group and a mercaptogroup, and X is a hydrolysable group such as an alkoxy group.Accordingly, examples of a suitable silane coupling agent includeaminosilane coupling agents, epoxysilane coupling agents, vinylsilanecoupling agents and methoxy- (ethoxy-) silane coupling agents, althoughsuitable silane coupling agents to be used are not restricted to thosementioned above.

The entire surface of the stainless steel substrate is usually coatedwith a silane coupling agent prior to laminating the transparent resinfilm. The silane coupling agent is dried using conventional particles.The silane coupling agent can be coated by, for example, brush coating,dip coating, spray coating or the like procedure using a solution of theagent. Specifically, for example, the silane coupling agent is dissolvedin, for example, alcohol to give a solution containing 1 to 5% of theagent, and the stainless steel substrate is coated therewith by such acoating method as mentioned above. Moreover, the following procedure mayalso be conducted: a silane coupling agent is impregnated with such acoating solution, and the stainless steel substrate is wiped with thecoupling agent to be coated therewith. Although the coating amount ofthe silane coupling agent during coating can be widely changed dependingon a desired treatment effect, the coating amount is usually from about1 100 mg/m² to 1,000 mg/m², preferably from about 10 100 mg/m² to 100mg/m². The silane coupling agent subsequent to coating may be subjectedto natural drying. However, it is preferred to dry the agent by heatingat temperatures of about 40° C. to 20° C. for several seconds to severalminutes. Specifically, for example, the stainless steel sheet is placedin an oven with internal air circulation, and it can be dried at about150° C. for 3 minutes, or at about 200° C. for 2 minutes.

The transparent resin-coated stainless steel article of the presentinvention may have an additional layer or film for improving theproperties and the appearance in addition to the stainless steelsubstrate and the transparent resin film. For example, in order toimprove the design of the article of the present invention, thetransparent resin film may also be printed with a pattern, letters orlogos, or a printed film may also be laminated to the transparent resinfilm. Furthermore, the surface of the resin film may also be coated witha transparent clear coating.

In the transparent resin-coated stainless steel article of the presentinvention, a total thickness of the transparent resin film laminated toa stainless steel substrate, i.e., its thickness including the thicknessof the applied silane coupling agent, is preferably in a range of about35 μm to 1,500, more preferably about 50 μm to 500. The thickness below35 μm is liable to deteriorate the light fastness and the durability,and the thickness above 1,500 μm is liable to deteriorate theappearance, along with deterioration of workability.

The transparent resin-coated stainless steel article of the presentinvention can be produced by various technologies, and there is nospecific limitation on the production method. In order to understandbetter the present invention, one embodiment will be explained. Forexample, a center pillar as an automotive exterior part can be producedby the following procedure.

A stainless steel sheet having a predetermined size is prepared. Foreignmaterials etc. sticking to the surface are removed by washing the steelsheet with water, and the steel sheet is degreased with an organicsolvent such as methyl ethyl ketone (MEK) or alcohol. The stainlesssteel sheet having been surface cleaned is entirely coated with asuitable silane coupling agent in the form of a solution. The coatingsolution of the silane coupling agent is an alcohol solution containingfrom about 1 to 5% thereof. The silane coupling agent, waste or thelike, is impregnated with the coating solution, and the entire surfaceof the stainless steel sheet is wiped with the coupling agent. In orderto bake the coating film of the solution of a silane coupling agentsubsequently to coating, the coating film is dried at 200° C. for 2minutes in an oven with internal air circulation.

When the primer treatment using a silane coupling agent is completed asexplained above, a transparent resin film is laminated to the stainlesssteel sheet (the silane coupling agent-treated surface) through atransparent heat-sensitive adhesive layer. For example, a composite filmcomprising a transparent urethane film and a transparent urethaneheat-sensitive adhesive layer can be used as the transparent resin film.

When laminating the resin film to the stainless steel sheet iscompleted, the stainless steel sheet held by a jig is press formed. Acenter pillar having a desired shape can thus be obtained withoutinvolving peeling and break of the resin film laminated to the stainlesssteel sheet. Since the bonded surface between the center pillar and theresin film thereon is excellent in water resistance, light resistance,etc., the properties required of an automotive exterior part can besufficiently satisfied. Moreover, even when the center pillar is exposedto harsh conditions over a long period of time, no inconvenience such aspeeling of the resin film occurs. Furthermore, since application of thesilane coupling agent can be conducted by a procedure similar to that ofapplying a conventional acrylic primer, the advantage ofmass-productivity brought by the use of the film is not lost. Stillfurthermore, since forming an article having a desired shape can beconducted after finishing a series of treatments including lamination ofthe resin film, the number of the steps can be reduced in the productionof the articles. Thus the yield of the articles can be increased incomparison with the conventional procedure comprising coating steelsubstrates with a resin after forming articles. Furthermore, since theresin film with which the stainless steel sheet is coated istransparent, manifestation of such properties of the stainless steelground as luster and metallic appearance is not impaired.

It will be understood from what is explained above that the transparentresin-coated stainless steel articles of the present invention can beadvantageously used for the production of various stainless steelarticles. Moreover, since the resin-coated stainless steel articles areparticularly excellent in water resistance, light resistance, or thelike, they can be especially advantageously used as automotive exteriorparts. Examples of the automotive exterior parts include grilles, trims,garnishes of door mirror and the like, in addition to the moldings andpillars described above.

EXAMPLES

The present invention will be subsequently explained in detail by makingreference to examples. In addition, it should be understood that thepresent invention is in no way restricted to the following examples.

Examples 1 to 6

Different silane coupling agents described below were used in respectiveexamples to produce transparent resin-coated stainless steel articles ofthe present invention as samples.

Example Silane coupling agent Trade name (manufacturer) 1 Aminosilanecoupling agent AP-133 (Nippon Unicar Company Limited) 2 Mercaptosilanecoupling APZ-730 (Nippon Unicar agent Company Limited) 3Aminopropyltriethoxy- A-1100 (Nippon Unicar silane Company Limited) 4Glycidoxypropyltri- A-187 (Nippon Unicar methoxysilane Company Limited)5 Glycidoxypropyltri- Sila-Ace S 510 methoxysilane (Tisso Corporation) 6Glycidoxypropyltri- Primer SCP 5 methoxysilane + (Tisso Corporation)acrylsilicone

First, a stainless steel sheet (Product name: SUS 430BA, manufactured byNisshin Seijou Co.) having a 150 mm long, 35 mm wide and 0.5 mm thickwas prepared. The surface was decreased with methyl ethyl ketone (MEK).A silane coupling agent was then impregnated with an ethanol solutioncontaining 5% of a silane coupling agent, and the surface of thestainless steel sheet was wiped with the silane coupling agent. Thestainless steel sheet was placed in an oven with internal aircirculation at 200° C., and dried for 2 minutes.

When the primer treatment with the silane coupling agent was completed,a transparent composite film was laminated to the stainless steel sheetwhile the treated surface was still at 200° C. The composite film usedherein was prepared by coating one side of a transparent polyurethanefilm (two solution type) 60 μm thick with a polyurethane heat-sensitiveadhesive (trade name of Desmocoll 530, manufactured by Bayer) at athickness of 25 μm. In order to firmly bond the composite film to thestainless steel sheet, the film was laminated to the steel sheet whilethe film was being pressed with a jig.

The stainless steel sheet having the composite film was subsequentlypress formed to give a sample. No peeling of the composite film wasobserved in the course of press forming of the sample.

Evaluation Test

The appearance and adhesive strength of the composite films of thesamples prepared in respective Examples 1 to 6 were evaluated underdifferent conditions (after application of warm water and afteraccelerated weathering test). The procedure of evaluating an adhesivestrength under each condition is shown below.

1. After Application of Warm Water

In the test, how the appearance of a composite film varies when warmwater is applied is evaluated along with water resistance of theadhesive strength of the film. A sample is immersed in warm water at 40°C. for 240 hours, taken out of the warm water bath, and allowed to standfor one hour. The composite film is peeled off the stainless steel sheetwith hands, and the peelable degree of the composite film is evaluatedat the following three levels:

γ . . . difficult to peel (adhesive strength clearly satisfying thestandard for automotive exterior films);

Δ . . . easily peelable (adhesive strength clearly not satisfying thestandard for automotive exterior films); and

× . . . unbonded and easily peelable.

In addition, when an extraordinary appearance is observed, theobservation is noted.

2. After Accelerated Weathering Test

In the test, how the adhesive strength of a composite film varies whenthe film is irradiated with outdoor light, namely, the light resistance(weathering resistance) of the adhesive strength is evaluated. A sampleis attached to an accelerated weathering testing machine (sunshinecarbon weatherometer), exposed to severe light irradiation, and takenout of the testing machine. The sample is allowed to stand for one hour,and the peelable degree of the composite film is evaluated at threelevels by the same procedure as mentioned above.

Evaluation results as described in Table 1 were obtained.

TABLE 1 Adhesive strength Adhesive strength Example (water resistance)(light resistance) 1 γ γ 2 γ γ 3 γ γ 4 γ γ 5 γ γ 6 γ γ

It is understood from the evaluation results in Table 1 thatsatisfactory adhesion strength sufficient to satisfy the requirements ofthe standard for automotive exterior films with regard to the waterresistance and light resistance could be obtained in Examples 1 to 6.

Comparative Examples 1 to 6

The procedures described in Examples 1 to 6 were repeated. However, inthe present comparative examples, the following procedures were employedfor comparison in place of the primer treatments using silane couplingagents. That is, a primer treatment was omitted, or primer treatmentswere conducted with different finishing agents as, described below. Inaddition, each finishing agent was usually commercially available as anadhesive.

Comp. Example Finishing agent Trade name (manufacturer) 1 No (stainlesssteel sheet) 2 Acrylic finishing agent Hamatight Y 1104B (YokohamaRubber Co., Ltd.) 3 Acrylic/modified phenolic KBS 17 (KONISHI CO., LTD.)finishing agent 4 Sodium hydroxide, KBS 17 5 Polyester/polyisocyanate Y6171 (KONISHI CO., LTD.) finishing agent 6 Aqueous acrylic finishing JA7454 (Sumitomo 3M Ltd.) agent 7 Polyamide finishing agent PROMOTOR 86(Sumitomo 3M Ltd.) 8 Phenol/polyvinyl butyrate JA 7413R (Sumitomo 3MLtd.) finishing agent

In the table described above, Comparative Example 1 is an instance inwhich a sample was prepared without surface treatment of the stainlesssteel sheet. Comparative Examples 2, 3, 5 to 8 correspond to Examples 1to 6, respectively. Although Comparative Example 4 corresponds toExample 3, the stainless steel sheet surface was first wiped to coat thesheet surface with an aqueous solution containing 5% of sodiumhydroxide, dried, and surface treated with KBS 17.

Evaluation results as described in Table 2 were obtained.

TABLE 2 Adhesive strength Adhesive strength Comp. Example (waterresistance) (light resistance) 1 x γ 2 γ x  80 hr 3 γ x  80 hr 4 x  30hr 5 Δ 1200 hr 6 x  80 hr 7 Δ 1200 hr 8 x  80 hr Note: Hr (time) in thecolumn of Adhesive strength (light resistance) designates an elapsedtime until the occurrence of inconvenience (peeling of film).

It is understood from the evaluation results in Table 2 that no adhesionstrength sufficient to satisfy the requirements of the standard forautomotive exterior films with regard to the water resistance and lightresistance could be obtained, if the film-coated stainless steel sheetis produced without applying a silane coupling agent to a surface of thesheet to be coated.

Example 7

Twenty-three types of samples in total were prepared, and the adhesivestrength (N/25 mm) of each sample was measured.

First, two types of stainless steel sheets (SUS 430BA and SUS 304BA,both manufactured by Nisshin Seikou Co.) having a 150 mm long, 35 mmwide and 0.5 mm thick were prepared. The surface of each sheet wasdegreased with methyl ethyl ketone (MEK).

Each sheet was subsequently wiped with a wiping cloth impregnated withan ethanol solution containing 5% of a silane coupling agent used inExamples 1 to 6 or a finishing agent used in Comparative Examples 2 to8, as shown in Table 3. The coating amount of each coupling agent wasfrom about 0.02 to 0.05 g/m² in terms of dried weight. The coatingamount of each finishing agent was from about 2 to 5 g/m² in terms ofdried weight. As shown in Table 3, such primer treatment was omitted forsamples 3, 14 and 21. The stainless steel sheets were placed in an ovenwith internal air circulation at 200° C., and dried for 2 minutes.

When the primer treatment with a silane coupling agent or finishingagent was completed, one of the following three types of transparentcomposite films was laminated to the stainless steel sheet. Thecomposite films used herein each have a two-layer structure, and thelayer structures are as described below.

Composite Film 1:

Base: two solution type transparent polyurethane film (60 μm thick)

Adhesive layer: polyurethane heat-sensitive adhesive (trade name ofDesmocoll 530, manufactured by Bayer, 25 μm thick)

Composite Film 2:

Base: two solution type transparent polyurethane film (60 μm thick)

Adhesive layer: polyester heat-sensitive adhesive (trade name of ElitelUE 3500, manufactured by Unitika Corporation, 25 μm thick)

Composite Film 3

Base: two solution type transparent polyurethane film (60 μm thick)

Adhesive layer: acrylic transparent pressure-sensitive adhesive (tradename of SK Dyne 1502, manufactured by Soken Kagaku K. K., 25 μm thick)

Composite Film 4:

Base: two solution type transparent polyurethane film (black, 60 μmthick)

Adhesive layer: polyurethane heat-sensitive adhesive (trade name ofDesmocoll 530, manufactured by Bayer, 25 μm thick)

For a composite film 3 in which a pressure-sensitive adhesive was used,the film was laminated to a stainless steel substrate while the film wasbeing pressed with a jig. Moreover, for a composite film 1 or 2 in whicha heat-sensitive adhesive was used, the composite film was laminated toa stainless steel sheet having been subjected to primer treatment andhaving temperature as high as 200° C.

Measurement of Adhesive Strength

The adhesive strength of the sample (hereinafter termed “specimen”)prepared as explained above was measured under different conditions(normal state, after application of warm water and after acceleratedweathering test). The methods of measuring adhesive strength underrespective conditions will be described below.

1. Normal State

In the test, the adhesive strength of a specimen is measured as apeeling strength (N/25 mm) obtained by a 180° peeling test of thespecimen 48 hours after preparing the specimen. In accordance with JIS Z6800, after attaching the specimen to a 180° peeling testing machine, aportion of the composite film 25 mm wide is removed by cutting, and theload is measured at the time when one end of the test piece is peeledoff in the 180° direction. Measurement results as described in Table 3were obtained.

Numerical values and abbreviations described in Table 3 are illustratedbelow.

Numerical value: peeling strength (N/25 mm)

F: lifting of the composite film during peeling test

C: cutting of the composite film during peeling test (the peelingstrength also being recorded)

NP: no peeling of the composite film during peeling test (the adhesivestrength being very high)

2. After Application of Warm Water

In the test, how the adhesive strength of a specimen varies when warmwater is applied, namely, the water resistance of the adhesive strengthis evaluated. The specimen is immersed in warm water at 40° C. for 240hours, taken out of the warm water bath, and allowed to stand for onehour. The peeling strength (N/25 mm) of the specimen is then measured asthe adhesive strength by a 180° peeling test. The method and conditionof measurement are the same as explained above. Measurement results asdescribed in Table 3 were obtained.

3. After Accelerated Weathering Test

In the test, how the adhesive strength of a specimen varies when thespecimen is irradiated with outdoor light, namely, the light resistance(weathering resistance) of the adhesive strength is evaluated. Thespecimen is attached to an accelerated weathering testing machine(sunshine carbon weatherometer), exposed to severe light irradiation for2,000 hours, and taken out of the testing machine. The specimen isallowed to stand for one hour, and the peeling strength (N/25 mm) ismeasured as the adhesive strength by a 180° peeling test. The method andcondition of measurement is the same as explained above. The measurementresults thus obtained are shown in Table 3.

TABLE 3 Spec- Adhesive Strength (N/25 mm) imen Type of Primer NormalWater No. SUS Film treatment state res* Light res* 1 430 BA 4 KBS 17 NP69 C. NP 2 430 BA 1 KBS 17 NP 62 C. 1 3 430 BA 1 — 55 F 29 4 430 BA 1 AP133 NP 56 C. 45 C. 5 430 BA 1 APZ 730 NP NP 40 C. 6 430 BA 1 A 1100 NPNP 40 C. 7 430 BA 1 A 187 NP 78 C. 35 C. 8 430 BA 1 Sila-Ace S 510 NP 76C. 46 C. 9 430 BA 1 Y 1104B NP NP F 10 430 BA 1 Y 6171 NP 8 6 11 430 BA1 JA 7454 NP 11 2 12 430 BA 1 PROMOTER 86 NP 4 6 13 430 BA 2 KBS 17 56C. 38 F 14 430 BA 2 — 25 1 27 15 430 BA 2 AP 133 51 38 55 C. 16 430 BA 2A 1100 51 C. 44 C. 39 C. 17 430 BA 3 AP 133 41 34 25 18 430 BA 3 A 110040 33 27 19 304 BA 4 KBS 17 NP 55 C. 50 C. 20 304 BA 1 KBS 17 70 C. 42 F21 304 BA 1 — 73 C. 1 43 C. 22 304 BA 1 AP 133 65 35 38 23 304 BA 1 A1100 53 C. 37 40 Note: res* = resistance

It is understood from the measurement results described in Table 3 thateffective adhesion strength could be obtained only when a silanecoupling agent was applied to a surface of the stainless steel sheet tobe coated with a film. Further, effective adhesion strength could beobtained when the stainless steel sheet was changed or an adhesive layerof the film was changed.

Example 8

In the present example, how the adhesive strength (N/25 mm) of a samplevaries when the coating amount of a silane coupling agent is changed wasevaluated.

Specimen 4 (aminosilane coupling agent AP-133 being used) and specimen 6(aminopropyltriethoxysilane A-1100 being used) in Example 7 were used,and the procedures described in Example 7 were repeated. The coatingamounts of AP-133 or A-1100 were then changed to 10, 1, 0.1, 0.01, 0.001or 0.0001 g/m² as described in Table 4. Measurement results as describedin Table 4 were obtained.

TABLE 4 Silane coupling agent Adhesive strength (N/25 mm) Specimen No.Type Coating amount (g/m²) Normal Water res* Light res* 4A AP 133 10 69C. 1 22 4B AP 133 1 NP 16 35 4C AP 133 0.1 37 C. 44 C. 53 4D AP 133 0.0142 C. 62 C. 50 4E AP 133 0.001 51 C. 22 43 4F AP 133 0.0001 49 C. 1 306A A 1100 10 73 C. 1 33 6B A 1100 1 41 C. 22 44 6C A 1100 0.1 NP 35 406D A 1100 0.01 72 C. 38 35 6E A 1100 0.001 68 C. 19 36 6F AP 133 0.000151 C. 2 29 Note: res* = resistance

It is understood from the measurement results in Table 4 that a coatingamount of the silane coupling agent is effective in the range of 1 to1,000 mg/m², preferably in the range of 10 to 100 mg/m².

EFFECT OF INVENTION

As explained above, the present invention provides a stainless steelarticle which has a transparent and uniform resin coating thereon, whichcan utilize the surface appearance of the stainless steel used as asubstrate, which shows a high adhesive strength between the resincoating and stainless steel as a ground, which can be easily producedwith a small number of steps, and which is particularly useful as anautomotive exterior article.

1. A resin-coated stainless steel article, comprising a stainless steelsubstrate and a transparent resin film with which at least one surfaceof the substrate is coated, the transparent resin film comprising atransparent polyurethane base and a transparent heat-sensitive adhesivelayer fanned on the transparent polyurethane base on the side of thestainless steel substrate, and the transparent resin film being bondedto the stainless steel substrate through a silane coupling agent.
 2. Theresin-coated stainless steel article according to claim 1, wherein theentire surface of the stainless steel substrate is coated with thesilane coupling agent in a coating amount of 1 mg/m² to 1,000mg/m². 3.The resin-coated stainless steel article according to claim 1, whereinthe adhesive strength of the resin film with the stainless steelsubstrate is at least 14 N/25 mm in terms of peeling strength.
 4. Theresin-coated stainless steel article according to claim 1, wherein thetransparent heat-sensitive adhesive is a polyurethane adhesive, apolyester adhesive, an olefinic adhesive, an acrylic adhesive or a vinylchloride adhesive.
 5. The resin-coated stainless steel article accordingto claim 1, wherein the resin-coated stainless steel article is appliedto an automotive exterior.
 6. The resin-coated article according toclaim 1, wherein the synthetic resin base and the heat-sensitiveadhesive layer are transparent.
 7. A method of forming a resin-coatedstainless steel article, comprising applying a transparent resin filmcomprising a transparent polyurethane base and a transparentheat-sensitive adhesive layer to a stainless steel substrate, whereinsaid transparent resin film is bonded to a surface of said stainlesssteel substrate through a silane coupling agent.